Data transmission through any form of channel rarely operates in as ideal a state as in theory. To compensate the distortion caused by the transmission channel, adaptive equalizers have been used to facilitate data reception. The equalizers, which date back to the use of loading coils to improve the characteristics of twisted-pair telephone cables for voice transmission, become even more necessary in high-speed data transmission for reducing the intersymbol interference introduced by the channel.
A conventional method for equalizer train-up has been to use a pseudo-random transmitter training sequence, Known at the receiver, to correlate with the incoming received signal samples. Knowing the start-time of the training sequence, the receiver feeds this sequence along with the received samples to an adaptive feedforward equalizer.
The feedforward equalizer generally uses a "steepest-descent" algorithm to adapt its tap coefficients until the set of the equalizer coefficients have converged to a point where they equalize the distorted received samples ("equalizing response"). The resulting equalizing response is then positioned in the equalizer according to the relative delay between the received signal samples and receiver-generated training sequence. However, for badly distorted channels where the total number of equalizer taps (the "span") is only just sufficient to contain the equalizing response, centering of this response within the taps thus becomes crucial for good equalizer performance.
Furthermore, adaptive equalizers typically require a significant portion of the processing power to keep updating its tap coefficients. As such, systems have been implemented with the span demanded by the worst-case channel will demand. If the equalizing response is not correctly positioned within the equalizer's taps, either the system performance will be degraded or the equalizer will require more than the minimum number of taps.